A pair of flat grid blanks are clamped together at their edges with an impervious metal sheet on top. All of the blanks and sheets are dished simultaneously by forcing fluid to inflate an elastic sheet which contacts the bottom grid blank. A second impervious metal sheet is inserted between the two grid blanks if the grids have high percentage open areas. The dished grids are stress relieved simultaneously

Banks, B. A.

English

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fc 9 NATIONAL AERONAUTICS AND SPACE A D M I N I S T R A T I O N
gilt' WASHINGTON, D.C. 20546
F WAR 7 "375
GP
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U.S. Patent No. : Y ?" k 7 ,*7 ^^
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United States Patent
<•
Banks
[in 3,864,797
[45] Feb. 11, 1975
[54] METHOD OF MAKING DISHED ION
THRUSTER GRIDS
[75] Inventor: Bruce A. Banks, Olmsted, Ohio
[73] Assignee: The United States of America as
represented by the Administrator of
the National Aeronautic and Space
Administration, Washington, D.C.
[22] Filed: Apr. 18, 1973
[21] Appl. No.: 352,381
[52] U.S. Cl 29/25.18, 72/63
[51] Int. Cl HOlj 9/02
[58] Field of Search 29/25.18, 25.14,25.17;
72/63
[56} References Cited
UNITED STATES PATENTS
1,329,969 2/1920 Harrison 72/63
OTHER PUBLICATIONS
David C. Byers, "An Experimental Investigation of a
High-Voltage Electron-Bombardment Ion Thruster,'
Jan. 1969, J. Electrochem Soc. pp. 9-17.
Primary Examiner—Granville Y. Custer, Jr.
Assistant Examiner—James W. Davie
Attorney. Agent, or Firm—N. T. Musial; G. E. Shook;
J. R. Manning
[57] ABSTRACT
A pair of flat grid blanks are clumped together at their
edges with an impervious metal sheet on top. All of
the blanks and sheets are dished simultaneously by
forcing fluid to inflate an elastic sheet which contacts
the bottom grid blank. A second impervious metal
sheet is inserted between the two grid blanks if the
grids have high percentage open areas. The dished
grids are stress relieved simultaneously.
8 Claims, 3 Drawing Figures
/tr\30 x-34
' PATENTED FEIMI875- 3.864,797
FIG.I
FIG.2
22
28
FIG. 3
1
3,864,797
METHOD OF MAKING DISHED ION THRUSTER
GRIDS
ORIGIN OF THE INVENTION
The invention described herein was made by an em-
ployee of the United States Government and may be
manufactured or used by or for the Government with-
out the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
dom edge slippage which causes wrinkles to be pro-
duced in the dished grids.
Dishing can also be achieved by simply hydroforming
a single grid with no backing sheet. However, this re-
5 duces the amount of dish depth possible because cata-
strophic rupture failure occurs. This further prevents
the hydroforming of dished accelerator grids having
high open areas.
10 SUMMARY OF THE INVENTION
These problems have been solved by the present in-
This invention is concerned with forming curved vention wherein no female die is utilized. An assembly
grids for an ion thruster accelerator system. The inven-
 of grid blanks separated and covered by impervious
tion is particularly directed to the fabrication of metal sheets is placed on top of an elastic sheet. The as-
matched pairs of dished grids having high percentage 15
 semb|ed sheets are c|arnped at their edges to prevent
open areas. random slippage. The blanks are then dished by forcing
Electron bombardment ion thrusters are being con-
 fluid to inflate the elastic sheet which in turn forces
sidered for a variety of space missions for which the op-
 the impervious sheets and grid blanks to dish to their
timum specific impulse is between 2000 and 3000 sec-
 natura, contour whicn is approximately hemispheroi-
•onds. Achievement of this optimum specific impulse 20
 da|. The impervious sheet between the grid blanks pre-
with a thruster having a double grid accelerator system
 vents distortion caused by s|ight misalignment of the
at the desired thrust density and thruster efficiency re-
quires a screen grid that is less than 0.5 mm thick and
has a high open area fraction, preferably in excess of
0.7. A grid-to-grid spacing of about 0.25 to 0.75 mm 25
must be maintained over the entire grid system. The ion
extract ion system must also survive both the launch en-
vi ronment and repeated thermal cycling in space.
These factors combine to impose a difficult mechani-
 eraU)r jds havj hj h „, „ areas that arc
cat problem on the ion thruster accelerator system. •"' . . . . P. ." . r .
Techniques such as holding the grids in tension, sup-
porting the screen grid from the mercury vapor distrib-
utor manifold, adding stiffener ribs, and using inter-
electrode supports have been proposed for the ion ex-
traction systems of thrusters utilized in these missions. 35
However, individual problems have been encountered
with each of these techniques.
Accelerator systems having closely spaced grids that
have been dished to a hemispheroidal configuration are
holes in the screen and accelerator grids. The dished
grids are stress relieved simultaneously in matched
dies.
OBJECTS OF THE INVENTION
It is, therefore, an object of the present invention to
provide well matched pairs of dished screen and accel-
Another object of the invention is to provide an im-
proved process for producing well matched molybde-
num grids having space variations of less than 0.1 mm.
Another object of the invention is to provide a pair
of grids that are uniformly dished to a spherical config-
uration.
These and other objects of the invention will be ap-
parent from the specification which follows and from
used without grid supports to satisfy the requirements 40 the drawing wherein like numerals are used throughout
imposed by near term missions. To obtain uniform to ldentlfy llke Parts-
DESCRIPTION OF THE DRAWINGsmall grid spacing between 0.25 and 0.75 mm requires
that the contours of the adjacent surfaces of the screen
and accelerator grids be nearly identical. Any local ir- FIG. 1 is a vertical section view of apparatus con-
regularity must occur at the sameTocTtion"and'to'the 45 structed in accordance with the invention showing an
same degree in both grids. assembly of grid blanks and metal sheets prior to dish-
Fabrication techniques which dish both grids simulta- In8'
neously tend to minimize grid spacing variations. Nu- FIG 2 Is a vertical section view similar to FIG- l
merous methods have been employed in attempts to showing the assembled grid blanks and metal sheets
fabricate dished accelerator grid systems. These in- 50 after dishing; and
elude spinning, hot and cold pressing, and hammering. FIG- 3 is a vertical section view of an alternate em-
However, irregularities in the resulting grid contours
have been so severe that the grids could not be success-
fully used in a closely spaced system.
A hydroforming process has also been proposed for
fabricating the grids. This process consists of inflating
55
bodiment of the apparatus shown in FIGS. 1 and 2.
DESCRIPTION OF THE PREFERRED
EMBODIMENT
Referring now to the drawing, there is shown in FIG.
1 a screen grid 10 and an accelerator grid 12 in the
form of perforated sheets that are substantially flat
grids individually into a female cavity. In this process
each grid is dished separately, and irregularities in
thickness result in nonmatching contours in the screen
 6Q prior to dishing. The two perforated sheets are molyb-
grid and accelerator grid. This produces uneven spac- denum.
The screen grid 10 preferably has a thickness of less
than about 0.5 mm. This screen grid has a high open
ing between the grids which is undesirable because of
ion focusing and electrical breakdown problems.
Hydroforming into a female die results in dished grid area fraction, preferably in excess of 0.7.
contours which are different from the female die be-
 65 The accelerator grid 12 is usually thicker than the
cause of the spring back in the grid material. Such hy-
droforming is done with a minimal amount of edge
clamping of the grid to be dished. This produces a ran-
screen grid 10. Accelerator grids having thicknesses up
to about 0.75 mm have been satisfactory. The accelera-
tor grid 12 has less open area than the screen grid 10.
3,864,797
The percentage open area may be as low as 50% in
some accelerator grids.
The grid blanks 10 and 12 may be separated by a
metal sheet 14 that is impervious. A stainless steel sheet
14 having a thickness of about 0.4 mm has been satis-
factory.
A second impervious metal sheet 16 covers the accel-
erator grid 12 to prevent catastrophic rupture. The as-
sembly of grid blanks and metal sheets is placed on an
elastic sheet 18 which covers a rigid base plate 20. The
elastic sheet 18 may be either rubber or plastic.
A metal clamping ring 22 is placed on top of the solid
metal sheet 16 so that the ring 22 is in engagement with
a marginal peripheral surface portion of the metal sheet
16. A plurality of clamping bolts 24 engage both the
metal ring 22 and the base plate 20 to maintain the
clamping ring in engagement with the marginal periph-
eral surface portion of a top metal sheet 16. A protu-
berance on the ring 22 mates with an aligned depres-
sion in the base plate 20. This clamps the peripheral
edges of the grids and metal sheets to prevent random
slippage during dishing.
A centrally disposed passage 26 is placed in commu-
nication 'with a fluid line 28 by a fitting 30. The end of
lower grid 10 the metal sheet 14 between the grids can'"
be omitted'. Omission of the metal sheet 14 facilitates
the aligning of the holes in the two grids.
When the metal separating sheet 14 is used grid
5 alignment is achieved by first matching the holes in the
grids. The two grid sheets with holes aligned are then
placed on the metal sheet 14. A plurality of holes are
then drilled through all three assembled sheets. The im-
pervious metal sheet 14 is then inserted between the
10 grid sheets 10 and 12 with suitable fasteners being
passed through the match drilled holes to maintain the
alignment of the grid apertures.
15
DESCRIPTION OF AN ALTERNATE
EMBODIMENT
It is contemplated that an intermediate step can be
added by hydroforming with the two grids and metal
sheets into an intermediate female die 56. This die has
a geometry which more uniformly distributes the elon-
20 gallon as shown in FIG. 3.
After the intermediate step illustrated in FIG. 3 the
hydroforming is completed in the manner shown in
FIGS. 1 and 2. Stress relieving is performed after the
intermediate step shown in FIG. 3 without any matched
the line 28 opposite the fitting 30 is connected to a 25 dies. After the final dishing step shown in FIG. 2 stress
source 32 of high pressure fluid through a valve 34. The relieving is achieved with matched dies.
source 32 is pressurized by a pump which is not shown.
The fluid is preferably a l iquid, and satisfactory results
have been obtained when the source 32 contains water
under high pressure.
When the valve 34 is opened, high pressure hydraulic
liquid from the source 32 passes through the line 28
and passage 26 to the bottom surface of the elastic
sheet 18. This pressurized liquid inflates the sheet 18
against the bottom grid blank 10. The inflation of the
sheet 18 forces the sheets 14 and 16 and grid blanks 10
and 12 above it to dish to their natural contour as
shown in FIG. 2. This configuration is approximately
hemispheroidal.
During this dishing the solid metal sheet 14 between
the grids 10 and 12 prevents distortion caused by slight
misalignment of the holes in the screen and accelerator
grids. Such distortion occurs when the grids have high
percentage open areas because the metal between the
apertures in the screen grid 10 is forced into the open-
ings in the accelerator grid 12.
After the grids and sheets have been dished to the
configuration shown in FIG. 2 the fluid is removed
from the space between the deformable sheet 18 and
the base plate 20. The clamping 24 bolts are then re-
moved to release the metal ring 22. The sheets and
grids can then be removed from the base plate 20 and
stress relieved.
Stress relieving is achieved by heating the matched
pair of dished grids to a temperature of about 1,750° F
in matched dies. The grids maintained at this tempera-
ture in the matched dies for about 1 hour. During this
stress relieving an inert gas, such as helium or argon, is
provided to the space around the grids to prevent oxi-
dation.
It is pertinent to note that the impervious metal sheet
14 is only required when the grid blanks 10 and 12 have
high percentage open areas to prevent the metal of the
grid 10 from being forced into the openings in the grid
12. When the open area fraction is less than about 0.7
the separating sheet 14 need not be used. Also, if the
upper grid 12 is of a much lower open area than the
30
40
45
50
While several embodiments of the invention have
been illustrated and described it will be appreciated
that other modifications may be made without depart-
ing from the spirit of the invention or the scope of the
subjoined claims. By way of example, a photoresist can
be deposited on impervious grid blanks which are then
hydroformed in accordance with the invention. The re-
sulting dished blanks then have the holes chemically
etched followed by simultaneous stress relieving. When
the grids are dished in this manner prior to forming the
holes both metal sheets 14 and 16 can be omitted. This
embodiment is used when the spring back variation due
to differences in open areas must be eliminated. This
embodiment is also used where discontinuities in the
open areas might cause tearing of the grids during hy-
droforming because of stress concentrations.
I claim:
1. A method of forming matched pairs of dished grids
to a substantially hemispheroidal configuration for ion
thruster accelerator systems comprising the steps of
placing an elastic sheet on a rigid base member,
aligning a pair of substantially flat grid blanks to form
an assembly with said elastic sheet,
clamping the marginal peripheral portions of said as-
sembly with a ring to prevent slippage of said as-
sembly and to form an expansible fluid chamber
between said elastic sheet and said rigid base mem-
ber,
applying a fluid pressure to said elastic material
thereby inflating the same whereby said blanks are
dished to the curvature of said inflated elastic ma-
terial,
removing the dished grids from said assembly, and
stress relieving said dished grids simultaneously.
2. A method of forming matched pairs of dished grids
as claimed in claim 1 including the step of
covering the aligned grid blanks with a substantially
flat sheet of impervious metal so that said impervi-
ous metal is in engagement with the other of said
grid blanks opposite the elastic material to prevent
catastrophic rupture during dishing.
3,864,797
j 3J A method of forming matched pairs of dished grids
as claimed in claim 2 including the step of
interposing a second substantially flat sheet of imper-
vious metal between the grid metal blanks to pre-
vent distortion caused by slight misalignment of ap-
ertures in the grids during dishing.
4. A method of forming matched pairs of dished grids
as claimed in claim 1 wherein at least one of the grid
blanks has a thickness less than 0.5 millimeter.
5. A method of forming matched pairs of dished grids
as claimed in claim 4 wherein the grid blank is molyb-
denum.
10
6. A method of forming matched pairs of grids as
claimed in claim 1 including stress relieving the grids
after dishing by heating the same to a temperature of
about 1,750° F for about 1 hour.
7. A method of forming matched pairs of grids as
claimed in claim 1 including the step of
forming holes in the grid blanks after dishing.
8. A method of forming matched pairs of grids as
claimed in claim 7 wherein the holes are formed by
chemical etching.
15
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Method of making dished ion thruster grids

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